In many processes, such as distillation or direct contact cooling, it is advantageous to use structured packing to promote heat and mass transfer between liquid and vapor streams which flow countercurrently to each other inside a column. Structured packing offers the benefit of high efficiency for heat and mass transfer combined with low pressure drop, when it is compared with dumped or random packing or with distillation trays. The most commonly used structured packing consists of corrugated sheets of metal or plastics foil or corrugated mesh cloths stacked vertically. These foils have various forms of apertures and/or surface roughening features aimed at improving the heat and mass transfer efficiency. While there are many types of structured packing described in the prior art, some of which are discussed below, they have evolved from a variety of applications and most have not been developed or optimized specifically for cryogenic separation processes, such as those used for separating the components of air.
U.S. Pat. No. 4,296,050 (Meier) describes the use of the combination of apertures with fluting or grooves in a structured packing. The fluting runs at an angle of 15.degree.-90.degree. to the vertical, in a direction generally opposed to that of the corrugations, which run at an angle of 15.degree.-60.degree. to the vertical. The wavelength of the fluting is in the range of 0.3 to 3.0 mm. The packing has widespread application in a variety of heat and mass transfer processes.
U.S. Pat. No. 4,186,159 (Huber) discloses a structured packing having alternating bands of fluted and unfluted (plain) regions which run horizontally across the packing when it is viewed in its intended orientation within the column. The extent of these bands is at least 5 mm. The open area is specified to be 5-20%.
U.S. Pat. No. 4,455,339 (Meier) describes the use of alternating corrugated and uncorrugated portions within each sheet of structured packing. The liquid acceleration, which would occur in the uncorrugated portions, is claimed to improve mass transfer performance.
U.S. Pat. No. 4,597,916 and U.S. Pat. No. 4,604,247 (Chen et al.) describe the use of crisscrossing patterns produced by expanded metal. They also show the use of perforations in combination with the crisscrossing patterns or horizontal slits. The use of perforated sheets alternating with corrugated sheets, both with a variety of holes and features, is also disclosed.
EP-A-337150 (Lockett et al.) describes the use of especially deep fluting which is said to improve the mass transfer performance due to increased liquid hold up and better spreading.
U.S. Pat. No. 4,981,621 (Pluss) describes the use of crisscrossing texture without holes, which improve the liquid spreading.
U.S. Pat. No. 5,132,056 (Lockett et al.) describes the use of edge modification to improve wetting, especially under turndown conditions.
U.S. Pat. No. 5,454,988 (Maeda) discloses the use of special fluting in a corrugated packing with no holes. The fluting generally runs in a horizontal direction and is more square-wave like than sine-wave like in cross-section. The fluting also has a meandering flow path for laterally spreading liquid.
It is also well-known in the prior art that mesh type packing helps spread liquid efficiently and gives good mass transfer performance, but mesh type of packing is much more expensive than most of the foil type packing described above.
An object of the present invention is to provide a specific structure that shows high performance characteristics for cryogenic applications such as those used in air separation.
A further object of the present invention is to generalize the specific structure such that it also shows a high performance in other, and preferably all, heat and mass transfer applications.